Well, May and Dawn was used in the anime, but that is not as a protagonist, but a secondary main character, a traveling companion for Ash.
I admit, that Ash's journey following the same pattern of travel to new region, badge quest for the regional Pokemon League, mentally and physically reset to an amateur, then lost the league and restart the cycle. Despite that Ash get a new outfit for each region, the basic idea is still the same that Ash "start out fresh" in each region. So if it is "start out fresh", then it will certainly be much better that a new protagonist is used rather then reusing Ash again, because that is then the real "start out fresh" for the Pokemon anime as well.
One suggested that Ash was used continuously because anime afraid to lose viewer. But ironically, despite that Pokemon anime remains to have high view rate among other animes in Japan, but the overall Pokemon anime view rate within itself was going down for every generation. What they was afraid was becoming true.
One suggested that Ash was not replaced because the anime producer thought that viewers were "attached" to the protagonist, but once an anime became a long term anime over +15 years, the emotional attachment issue became rather very vague. Because Ash was based on the game protagonist of a game of 16 years ago, where nowaday, very few children of less then 15 years old knew about RGB on Gameboy, probably they won't even knew about it unless GF made the RGB remake now know as FrLg. So I don't know how much of the viewers are still "attached" to Ash Ketchum, but one much for sure is that such emotional attachment will definitely fade away as time goes on.
One suggested that Ash was not replaced because he (and Pikachu as well) had became the heart and core of the anime. Well, that is technically true, because he is the protagonist (and Pikachu is the protagonist's first Pokemon). But being a protagonist just means a story is told from his experience, that doesn't mean the Pokemon franchise as a whole lives on Ash Ketchum's existence. The Pokemon anime is to advertise the game, not Ash's story, so it really doesn't matter is the Pokemon anime told from Ash's experience or not.
In some other thread I read before, someone suggested that Ash was not replaced because the anime had became so popular that ending the story will sadden the viewers. What I personally wanted to say is that such statement is just a hilarious wishful thinking. Some of the long term anime like Doraemon and Dragonball, the former one was a DAILY sci-fi story, so the genre itself can already sustain long term story even without progress; the latter one was fantasy action, but characters will grow, so the anime itself grow as well as in the same pace of the viewer, so viewers are less likely to leave the show despite that story last for more than 10 years. If it is made into a series, I would rather use some successful examples like Kamen Rider series and Gundam series. Especially Gundam, where it had became the classic and representative of mecha animes, where it is so popular that in Japan when you talk about robots, Gundam is the first thing pop up in ones' mind. The first Gundam anime was in 1979, up until now new series was still making, and new games, toys, plan models, fan books, illustration books, all sorts of merchandises are sold with nice sale record even by now. And at 2009, a 1:1 Gundam Model (specifically the first Gundam in 1979) was built to celebrate its 30th anniversary at Odaiba. Gundam's popularity was not decreased because there were freshness in its new anime for every 1~2 years.
The only kind of reason I saw (personal opinion in here, so don't be angry if one dislike it) was merely the financial and marketing reason, and probably a little bit of pretentiousness as a marketer. What I heard back in the past OS generation, was that TV Tokyo was tending to extend the Pokemon anime because it has such high viewer rate, despite the fact that Shudo Takeshi wanted to end it. So the anime producer and broadcaster was dwelling on the sweetness of the fruit of past high view rate. And as the Pokemon franchise became famous, the anime producer had made a common mistake of most company that grows large: pretentiously assumption, especially assuming that audience of Pokemon anime were hoping for a "children anime", where every single "dark" elements were tone down to the point of uninteresting. Now, although the Pokemon anime with Ash Ketchum remain as the protagonist still running, but the view rate going down is already suggesting that the show is not live on by the quality of the show itself, but due to the fame of Pokemon franchise itself.
But well, I understand that my opinion is a difficult argument for being the only reason of why Ash was not replaced with other game protagonist. Doesn't matter will then the story follow exactly or partially adapt from the game, or even it is the same badge quest with other protagonist, but at least replacing Ash with another character is a much better option in my opinion.
I don't know what were the Pokemon anime scriptwriter(s), and more importantly the script director was thinking. If there is ever a chance, I just wanted to dump all these question regarding on the Pokemon anime to the scriptwriting team.
